U.S. patent application number 13/378038 was filed with the patent office on 2012-04-19 for method and apparatus for data transmission based on distributed discrete power control in cooperative multi-user multi-input multi-output system.
This patent application is currently assigned to INDUSTRY UNIVERSITY COOPERATION FOUNDATION, SOGANG UNIVERSITY. Invention is credited to Jaewon Chang, Jun-Ho Jo, Byoung-Hoon Kim, Jaewon Kim, Ki Jun Kim, Dong-Uk Lee, Wonjin Sung.
Application Number | 20120093250 13/378038 |
Document ID | / |
Family ID | 43411194 |
Filed Date | 2012-04-19 |
United States Patent
Application |
20120093250 |
Kind Code |
A1 |
Kim; Byoung-Hoon ; et
al. |
April 19, 2012 |
METHOD AND APPARATUS FOR DATA TRANSMISSION BASED ON DISTRIBUTED
DISCRETE POWER CONTROL IN COOPERATIVE MULTI-USER MULTI-INPUT
MULTI-OUTPUT SYSTEM
Abstract
Disclosed are a method and apparatus capable of enhancing a
closed loop multi-input multi-output (MIMO) capacity through
distributed discrete power control in the case of cooperatively
transmitting information to multiple users through a downlink.
Inventors: |
Kim; Byoung-Hoon; (
Gyeonggi-Do, KR) ; Sung; Wonjin; (Seoul, KR) ;
Chang; Jaewon; (Seoul, KR) ; Jo; Jun-Ho; (
Gyeonggi-Do, KR) ; Lee; Dong-Uk; (Seoul, KR) ;
Kim; Jaewon; (Seoul, KR) ; Kim; Ki Jun;
(Seoul, KR) |
Assignee: |
INDUSTRY UNIVERSITY COOPERATION
FOUNDATION, SOGANG UNIVERSITY
Seoul
KR
LG ELECTRONICS INC.
Seoul
KR
|
Family ID: |
43411194 |
Appl. No.: |
13/378038 |
Filed: |
November 17, 2009 |
PCT Filed: |
November 17, 2009 |
PCT NO: |
PCT/KR2009/006737 |
371 Date: |
December 13, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61221548 |
Jun 29, 2009 |
|
|
|
Current U.S.
Class: |
375/267 |
Current CPC
Class: |
H04B 7/0404 20130101;
H04W 52/247 20130101; H04B 7/0639 20130101; H04W 52/40 20130101;
H04W 52/241 20130101 |
Class at
Publication: |
375/267 |
International
Class: |
H04B 7/02 20060101
H04B007/02 |
Claims
1. A method for data transmission based on distributed discrete
power control in a closed loop cooperative multi-user multi-input
multi-output (MIMO) system, the method comprising: coding and
modulating a signal to be transmitted; receiving feedback
information from a receiver; sharing a pre-coding index with
receivers grouped for multi-user transmission, based on the
feedback information; determining a distributed discrete power
control level based on the feedback information; and transmitting,
to the receiver, the signal to be transmitted having the determined
distributed discrete power control level.
2. The method of claim 1, wherein the distributed discrete power
control level is determined by performing Hadamard product with
respect to a transmission signal and a distributed discrete power
control matrix.
3. The method of claim 2, wherein the feedback information is a
reception Signal to Interference-plus-Noise Ratio (SINR) of a
terminal, and a pre-coding index of a pre-coding book of the
terminal.
4. The method of claim 3, wherein the distributed discrete power
control matrix is determined based on at least one of: a method for
allocating much power to a base station having high signal power to
be received by each terminal; and a method for allocating the same
signal power to each base station by requesting a base station
having low signal power so as to have increased signal power.
5. The method of claim 3, wherein the pre-coding book is a Discrete
Fourier Transformation (DFT) based codebook.
6. A base station for data transmission based on distributed
discrete power control in a closed loop cooperative multi-user
multi-input multi-output (MIMO) system, the base station
comprising: a first processor configured to perform coding and
modulating processes with respect to a signal to be transmitted; a
receiver configured to receive feedback information from a
terminal; a second processor configured to determine a distributed
discrete power control level of the transmission signal based on
the feedback information; and a transmitter configured to transmit
the transmission signal.
7. The base station of claim 6, wherein the feedback information is
a reception Signal to Interference-plus-Noise Ratio (SINR) of the
terminal, and a pre-coding index of a pre-coding book of the
terminal.
8. The base station of claim 7, wherein the distributed discrete
power control matrix is determined based on at least one of: a
method for allocating much power to a base station having high
signal power to be received by each terminal; and a method for
allocating the same signal power to each base station by requesting
a base station having low signal power so as to have increased
signal power.
9. The base station of claim 7, wherein the pre-coding book is a
Discrete Fourier Transformation (DFT) based codebook.
10. A method for data transmission based on distributed discrete
power control in a closed loop cooperative multi-user multi-input
multi-output (MIMO) system, the method comprising: determining
feedback information; transmitting the determined feedback
information to a transmitter; determining a distributed discrete
power control level based on the feedback information; and
transmitting a signal having the determined distributed discrete
power control level to a terminal.
11. The method of claim 10, wherein the distributed discrete power
control level is determined by performing Hadamard product with
respect to a transmission signal and a distributed discrete power
control matrix.
12. The method of claim 11, wherein the feedback information is a
reception Signal to Interference-plus-Noise Ratio (SINR) of the
terminal, and a pre-coding index of a pre-coding book of the
terminal.
13. A terminal for data transmission based on distributed discrete
power control in a closed loop cooperative multi-input multi-output
(MIMO) system, the terminal comprising: a first processor
configured to determine feedback information to be transmitted to a
transmitter; a second processor configured to determine a
distributed discrete power control level based on the feedback
information; and a transmitter configured to transmit the feedback
information to a base station, and transmit a signal to a base
station based on the determined distributed discrete power control
level.
14. The terminal of claim 13, wherein the distributed discrete
power control level is determined by performing Hadamard product
with respect to a transmission signal and a distributed discrete
power control matrix.
15. The terminal of claim 14, wherein the feedback information is a
reception Signal to Interference-plus-Noise Ratio (SINR) of the
terminal, and a pre-coding index of a pre-coding book of the
terminal.
Description
TECHNICAL FIELD
[0001] The present invention relates to a wireless communication,
and particularly, to a method and apparatus capable of enhancing a
downlink transmission function through distributed discrete power
control in a multi-input and multi-output (MIMO) system.
BACKGROUND ART
[0002] The next generation mobile communication systems and
wireless transmission systems in multi-cell environments require an
enhanced data transmission rate and system capacity. Accordingly,
have been performed research for multi-input multi-output (MIMO)
systems capable of transmitting data by using a plurality of
antennas. Among the MIMO systems, a closed loop MIMO system
enhances a data transmission function by utilizing fed-back channel
information so as to enhance a data transmission rate in multi-cell
environments.
[0003] FIG. 1 is a view showing a multi-cellular mobile
communication system with consideration of cooperative transmission
between base stations.
[0004] Referring to FIG. 1, a first base station 30 communicates
with a first terminal 10, and a second base station 40 communicates
with a second terminal 20.
[0005] The first and second terminals 10 and 20 feedback their
channel information to one or more base stations 30 and 40.
Accordingly, the fed-back information is shared between the
respective base stations through a controller 50. Each station
transmits information to each terminal based on channel information
fed-back from the terminal. This may allow the base stations to
efficiently communication with the terminals.
[0006] FIG. 2 is a configuration view of a Closed Loop Multi-Input
Multi-Output (MIMO) system in accordance with the conventional
art.
[0007] Referring to FIG. 2, the base station 30 denotes a
transmitter, and the terminal 10 denotes a receiver.
[0008] The conventional closed-loop MIMO system comprises a base
station 30, a transmission antenna 31, a terminal 10, and a
reception antenna 11. And, a transmission signal is transmitted to
the terminal 10 after passing through an H matrix.
[0009] The base station 30 includes a coding and modulation unit 32
configured to coding and modulation processes with respect to a
transmission signal, a coding book 33, and a processor 34 for
multiplying one weight vector included in the coding book 33 by the
transmission signal.
[0010] The terminal 10 receives a signal transmitted from the base
station 30 through the reception antenna 11, thereby demodulating
the received signal by selecting one coding vector from the coding
book 12.
[0011] The terminal 10 feedbacks its channel information to the
base station 30, thereby allowing the base station 30 to
efficiently communicate with itself based on the channel
information.
[0012] Methods for enhancing a data transmission function by using
channel information by the closed-loop MIMO system include a
non-cooperative transmission method (1) and a cooperative
transmission method (2).
[0013] The non-cooperative transmission method denotes a method for
transmitting information, by a single base station, to a terminal
based on channel environments of the terminal within a cell
coverage. And, the cooperative transmission method denotes a method
for transmitting information, by a plurality of base stations, to a
plurality of terminals based on channel environments of the
terminals within each cell coverage.
[0014] Firstly, will be explained a Signal-to-Noise Ratio (SNR) of
the closed loop MIMO system based on a non-cooperative transmission
method.
[0015] In the closed loop MIMO system based on a non-cooperative
transmission method, an optimized system is designed without
considering channel environments of other terminals. However, in
the closed loop MIMO system based on a cooperative transmission
method, an optimized system is designed with consideration of
channel environments of other terminals. Accordingly, the
conventional system is designed in an assumption that each channel
is independently and identically distributed. The conventional
system has the following configuration.
[0016] N.sub.R.times.1 reception signal vectors received by a
terminal (receiver) having N.sub.R antennas from a base station
(transmitter) having N.sub.T antennas can be expressed as
follows.
r = Hx + n [ h 11 h 1 N T h N R 1 h N R N T ] [ W 1 i W N T i ] d +
[ n 1 n N R ] Equation 1 ##EQU00001##
[0017] Referring to the Equation 1, H denotes a channel matrix of
N.sub.R.times.N.sub.T, X is a transmission signal vector of
N.sub.T.times.1, and n denotes an additive white Gaussian noise
(AWGN) vector of N.sub.R.times.1. The transmission signal vector is
X=W.sub.id, which is obtained by multiplying a pre-coding vector
mapped to a codebook index (i) selected by the terminal from an
N.sub.T.times.L codebook matrix (W=[W.sub.1 . . . W.sub.L])
composed of L pre-coding vectors, by a transmission data symbol
(d).
[0018] W.sub.i for maximizing a reception signal strength at the
terminal is determined by the following Equation 2 based on the
aforementioned Equation 1. And, the terminal transmits a codebook
index (i) mapped to W.sub.i to the base station, thereby requesting
a pre-coding process.
i = argmax k = 1 , , L { Hw k 2 } Equation 2 ##EQU00002##
[0019] Once the resultant value obtained from the Equation 1 is
processed with a signal received by the terminal by using a
selected pre-coding vector, the following Equation 3 is
obtained.
( Hw i ) H r = Hw i 2 d + ( Hw i ) H n = .lamda. i d + ( Hw i ) H n
Equation 3 ##EQU00003##
[0020] .lamda..sub.i, denotes a beam-forming grain obtained by
using W.sub.i, and .differential..sub.n.sup.2 denotes power of an
additive white Gaussian noise (AWGN) component. In this case, a
Signal-to-Noise Ratio (SNR) can be expressed as the following
Equation 4.
.gamma. = Hw i 2 n 2 = .lamda. i .sigma. n 2 Equation 4
##EQU00004##
[0021] Next, will be explained the conventional closed loop MIMO
system based on a cooperative transmission method. In this case, B
base stations each having N.sub.T and K terminals each having
N.sub.R participate in a cooperative transmission method. Here,
N=BN.sub.T transmission antennas and M=KN.sub.R reception antennas
operate between the B base stations and the K terminals. And,
M.times.1 reception signal vectors received by the K terminals when
the B base stations transmit signals through the channel matrix
(M.times.N) can be expressed as the following Equation 5.
r ~ = H ~ x ~ + n ~ = H ~ ( W ~ d ~ ) + n ~ = [ H ~ 1 H ~ K ] [ w ~
1 w ~ K ] d ~ + n ~ = [ H 11 H 1 B H K 1 H KB ] [ w i 11 w i K 1 w
i 1 , B w i KB ] [ d 1 d K ] + [ n 1 n K ] Equation 5
##EQU00005##
[0022] Here,
[0023] {tilde over (H)}
[0024] denotes a channel matrix of M.times.N,
[0025] {tilde over (X)}
[0026] denotes a transmission signal vector of N.times.1, and
n
[0028] denotes an additive white Gaussian noise (AWGN) vector of
M.times.1.
[0029] {tilde over (W)}=[{tilde over (W)}.sub.1 . . . {tilde over
(W)}.sub.K]
[0030] denotes an M.times.K pre-coding matrix composed of
pre-coding vectors to be used between the B base stations and the K
terminals.
[0031] {tilde over (H)}.sub.m
[0032] is a N.sub.R.times.N channel matrix between the B base
stations and the m.sup.th terminal. And,
[0033] {tilde over (W)}.sub.m
[0034] denotes N.times.1 pre-coding vectors composed of B
pre-coding vectors used for the m.sup.th terminal by the B base
stations.
[0035] A channel (H.sub.m n) denotes a N.sub.R.times.N.sub.T matrix
between the n.sup.th base station and the m.sup.th terminal.
W.sub.i m n denotes N.sub.T.times.1 pre-coding vectors determined
to maximize a reception signal strength on the channel (H.sub.m n)
between the n.sup.th base station and the m.sup.th terminal.
d.sub.m denotes a data symbol to be transmitted to the m.sup.th
terminal, and n.sub.m denotes an additive white Gaussian noise
(AWGN) vector of N.sub.R.times.1 of the m.sup.th terminal. W.sub.i
m n for maximizing a reception signal strength on the channel
(H.sub.m n) between the n.sup.th base station and the m.sup.th
terminal is determined by the following Equation 6. And, the
terminal transmits a codebook index (i.sub.m n) mapped to W.sub.i m
n to the base station, thereby requesting a pre-coding process.
i mn = argmax k = 1 , , L { H mn w k 2 } Equation 6
##EQU00006##
[0036] A signal received by the m.sup.th terminal can be expressed
as the following Equation 7.
r m = H ~ m ( W ~ d ~ ) + n m = H ~ m [ w ~ 1 w ~ K ] d ~ + n m = H
~ m w ~ m d m + k = 1 , k .noteq. m K H ~ m w ~ k d k + n m
Equation 7 ##EQU00007##
[0037] When applying 1.times.N.sub.R reception signal process
vectors, u.sub.m=({tilde over (H)}.sub.m{tilde over
(w)}.sub.m).sup.H
[0038] , to the Equation 7 by using a selected pre-coding vector,
the following Equation 8 is obtained.
u m r m = H ~ m w ~ m 2 + d m + ( H ~ m w ~ m ) H ( k = 1 , k
.noteq. m K H ~ m w ~ k d k + n m ) = .lamda. m d m + n ' Equation
8 ##EQU00008##
[0039] .lamda..sub.m denotes a beam-forming gain obtained through a
cooperative transmission method. And, an SINR with consideration of
an additive white Gaussian noise (AWGN) signal and a multi-use
interference signal can be expressed as the following Equation
9.
.gamma. = H ~ m w ~ m 2 k = 1 , k .noteq. m K H ~ m w ~ m d k + n m
2 Equation 9 ##EQU00009##
[0040] That is, the conventional closed loop MIMO system based on a
cooperative transmission method and a non-cooperative transmission
method has capacity shown in the Equations 1 and 5. However, it is
difficult to derive the Equation 5 in the case of the conventional
closed loop MIMO system based on a cooperative transmission method.
The reason is because each channel is not independently and
identically-distributed in a cooperative transmission method while
each channel is independently and identically-distributed in a
non-cooperative transmission method. Accordingly, the conventional
closed loop MIMO system is not suitable for the cooperative
transmission method.
[0041] In order to solve this problem, have been required
techniques for enhancing a data reception function and signal
quality in non-identically distributed channel environments, and
capable of effectively applying the techniques to cellular
systems.
DISCLOSURE OF INVENTION
Solution to Problem
[0042] Therefore, an object of the present invention is to provide
a method and apparatus for data transmission based on distributed
discrete power control in a closed loop cooperative multi-input
multi-output (MIMO) system, capable of enhancing a signal reception
function and signal quality by optimizing reception signal quality
in non-identically distributed channel environments.
[0043] Another object of the present invention is to provide a
method and apparatus capable of utilizing the conventional
cooperative transmission method and non-cooperative transmission
method by enhancing data transmission efficiency and by
implementing feedback signals having small number of bits.
[0044] Still another object of the present invention is to provide
a method and apparatus capable of efficiently performing data
communication by combining the conventional pre-coding technique
used in a cooperative transmission method, with a distributed
discrete power control method.
[0045] To achieve these and other advantages and in accordance with
the purpose of the present invention, as embodied and broadly
described herein, there is provided a method for data transmission
based on distributed discrete power control in a closed loop
cooperative multi-user multi-input multi-output (MIMO) system, the
method comprising: coding and modulating a signal to be
transmitted; receiving feedback information from a receiver;
sharing a pre-coding index with receivers grouped for multi-user
transmission, based on the feedback information; determining a
distributed discrete power control level based on the feedback
information; and transmitting, to the receiver, the signal to be
transmitted having the determined distributed discrete power
control level.
[0046] The distributed discrete power control level may be
determined by performing Hadamard product with respect to a
transmission signal and a distributed discrete power control
matrix.
[0047] The feedback information may be a reception Signal to
Interference-plus-Noise Ratio (SINR) of a terminal, and a
pre-coding index of a pre-coding book of the terminal.
[0048] The distributed discrete power control matrix may be
determined based on at least one of: a method for allocating much
power to a base station having high signal power to be received by
each terminal; and a method for allocating the same signal power to
each base station by requesting a base station having low signal
power so as to have increased signal power.
[0049] The pre-coding book may be a Discrete Fourier Transformation
(DFT) based codebook.
[0050] To achieve these and other advantages and in accordance with
the purpose of the present invention, as embodied and broadly
described herein, there is also provided a base station for data
transmission based on distributed discrete power control in a
closed loop cooperative multi-user multi-input multi-output (MIMO)
system, the base station comprising: a first processor configured
to perform coding and modulating processes with respect to a signal
to be transmitted; a receiver configured to receive feedback
information from a terminal; a second processor configured to
determine a distributed discrete power control level of the
transmission signal based on the feedback information; and a
transmitter configured to transmit the transmission signal.
[0051] The feedback information may be a reception Signal to
Interference-plus-Noise Ratio (SINR) of the terminal, and a
pre-coding index of a pre-coding book of the terminal.
[0052] The distributed discrete power control matrix may be
determined based on at least one of: a method for allocating much
power to a base station having high signal power to be received by
each terminal; and a method for allocating the same signal power to
each base station by requesting a base station having low signal
power so as to have increased signal power.
[0053] The pre-coding book may be a Discrete Fourier Transformation
(DFT) based codebook.
[0054] According to another aspect of the present invention, there
is provided a method for data transmission based on distributed
discrete power control in a closed loop cooperative multi-user
multi-input multi-output (MIMO) system, the method comprising:
determining feedback information; transmitting the determined
feedback information to a transmitter; determining a distributed
discrete power control level based on the feedback information; and
transmitting a signal having the determined distributed discrete
power control level to a terminal.
[0055] The distributed discrete power control level may be
determined by performing Hadamard product with respect to a
transmission signal and a distributed discrete power control
matrix.
[0056] The feedback information may be a reception Signal to
Interference-plus-Noise Ratio (SINR) of the terminal, and a
pre-coding index of a pre-coding book of the terminal.
[0057] To achieve these and other advantages and in accordance with
the purpose of the present invention, as embodied and broadly
described herein, there is still also provided a terminal for data
transmission based on distributed discrete power control in a
closed loop cooperative multi-input multi-output (MIMO) system, the
terminal comprising: a first processor configured to determine
feedback information to be transmitted to a transmitter; a second
processor configured to determine a distributed discrete power
control level based on the feedback information; and a transmitter
configured to transmit the feedback information to a base station,
and transmit a signal to a base station based on the determined
distributed discrete power control level.
[0058] The distributed discrete power control level may be
determined by performing Hadamard product with respect to a
transmission signal and a distributed discrete power control
matrix.
[0059] The feedback information may be a reception Signal to
Interference-plus-Noise Ratio (SINR) of the terminal, and a
pre-coding index of a pre-coding book of the terminal.
[0060] The foregoing and other objects, features, aspects and
advantages of the present invention will become more apparent from
the following detailed description of the present invention when
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0061] The accompanying drawings, which are included to provide a
further understanding of the invention and are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and together with the description serve to explain
the principles of the invention.
[0062] In the drawings:
[0063] FIG. 1 is a view showing a multi-cellular mobile
communication system with consideration of cooperative transmission
between base stations;
[0064] FIG. 2 is a configuration view of a Closed Loop Multi-Input
Multi-Output (MIMO) system in accordance with the conventional
art;
[0065] FIG. 3 is a configuration view of a system capable of
optimizing reception signal quality through distributed discrete
power control, in non-identically distributed channel environments,
according to the present invention;
[0066] FIG. 4 is a configuration view showing a method for
determining a distributed discrete power control level by a base
station according to one embodiment of the present invention;
[0067] FIG. 5 is a configuration view showing a method for
determining a distributed discrete power control level by a
terminal according to another embodiment of the present
invention;
[0068] FIG. 6 shows diagrams for determining a distributed discrete
power control level;
[0069] FIG. 7 is a view comparing MIMO system capacities according
to the position of a terminal according to the present invention;
and
[0070] FIG. 8 is a view comparing MIMO system capacities when two
terminals are constantly positioned within each coverage of two
base stations participating in cooperative transmission.
MODE FOR THE INVENTION
[0071] Description will now be given in detail of the present
invention, with reference to the accompanying drawings.
[0072] Hereinafter, preferred embodiments of the present invention
will be explained in more detail with reference to the attached
drawings. The same or similar components of one embodiment as or to
those of another embodiment will be provided with the same or
similar reference numerals, and their detailed explanations will be
omitted. And, if it is judged that detailed descriptions of the
related art are not within the range of the present invention, the
detailed descriptions will be omitted.
[0073] FIG. 3 is a configuration view of a system capable of
optimizing reception signal quality through distributed discrete
power control, in non-identically distributed channel environments,
according to the present invention.
[0074] Referring to FIG. 3, a Closed Loop Multi-Input Multi-Output
(MIMO) system according to the present invention comprises a
transmitter 300, a transmitting antenna 305, a receiving antenna
102, and a receiver 100. And, signals are transmitted to the
receiver 100 through a channel matrix (H matrix).
[0075] The transmitter may be a base station, and the receiver may
be a terminal.
[0076] The transmitter 300 includes a coding modulation unit 301
for coding and modulating a transmission signal, a coding book 302,
a first processor 303 for multiplying one weight vector included in
the coding book 302 by the transmission signal, and a second
processor 304 for performing Hadamard product with respect to the
multiplied signal and a distributed discrete power control
matrix.
[0077] In the closed loop MIMO system of the present invention, a
signal is coded and modulated, and is multiplied by a weight
vector. Then, the multiplied signal is transmitted to the receiver
100 after passing through a power control matrix and a channel
matrix (H matrix).
[0078] The receiver 100 demodulates the signal received from the
transmitter 300 by selecting one coding vector from the coding book
101.
[0079] And, the receiver 100 feedbacks the reception signal and
information relating to a channel status to the transmitter 300,
thereby enabling optimized communication.
[0080] Referring to FIG. 3, in the case that a plurality of base
stations transmit signals to a plurality of users in a cooperative
manner in multi-cell environments, a signal model may be shown in
the following equation 10 with consideration of a power control
matrix P and a weight vector W.sub.i.
r ~ = H ~ x ~ + n ~ = H ~ { ( W ~ P ) d ~ } + n ~ = [ H 11 H 1 B H
K 1 H KB ] { ( [ w i 11 w i K 1 w i 1 B w i KB ] [ p 11 p K 1 p 1 B
p KB ] ) [ d 1 d K ] } + [ n 1 n K ] Equation 10 : Hadamardproduct
##EQU00010##
[0081] Here,
[0082] {tilde over (H)}
[0083] denotes a channel matrix of M.times.N,
[0084] {tilde over (X)}
[0085] denotes a transmission signal vector of N.times.1, and
n
[0087] denotes Additive White Gaussian Noise (AWGN) of
M.times.1.
[0088] {tilde over (W)}
[0089] denotes a N.times.K pre-coding matrix composed of pre-coding
vectors to be used between B base stations and K terminals. And, P
is a Power Control (PC) matrix, which is multiplied by the
pre-coding matrix
[0090] ({tilde over (W)})
[0091] by using the Hadamard product.
[0092] For understanding of the signal model, it is assumed that a
base station has two antennas, and a terminal has two antennas.
That is, in the case of applying the equation 10 to a system model
in a M.times.N=4.times.4 channel where two base stations (B=2, each
base station having N.sub.T=2) and two terminals (K=2 each terminal
having N.sub.R=2) are implemented, the signal model of the present
invention can be expressed as the following equation 11.
r ~ = [ H 11 H 12 H 21 H 22 ] { ( [ w i 11 w i 21 w i 12 w i 22 ] [
p 11 p 21 p 12 p 22 ] ) [ d 1 d 2 ] } + [ n 1 n 2 ] = [ h 11 11 h
12 11 h 11 12 h 12 12 h 21 11 h 22 11 h 21 12 h 22 12 h 11 21 h 12
21 h 11 22 h 12 22 h 21 21 h 22 21 h 21 22 h 22 22 ] { ( [ w 1 i 11
w 1 i 21 w 2 i 11 w 2 i 21 w 1 i 12 w 1 i 22 w 2 i 12 w 2 i 22 ] [
p T 1 p 1 p T 1 ( 1 - p 1 ) p T 1 p 1 p T 1 ( 1 - p 1 ) p T 2 ( 1 -
p 2 ) p T 2 p 2 p T 2 ( 1 - p 2 ) p T 2 p 2 ] ) [ d 1 d 2 ] } + [ n
1 1 n 2 1 n 1 2 n 2 2 ] = [ h 11 11 h 12 11 h 11 12 h 12 12 h 21 11
h 22 11 h 21 12 h 22 12 h 11 21 h 12 21 h 11 22 h 12 22 h 21 21 h
22 21 h 21 22 h 22 22 ] [ p T 1 p 1 w 1 i 11 p T 1 ( 1 - p 1 ) w 1
i 21 p T 1 p 1 w 2 i 11 p T 1 ( 1 - p 1 ) w 2 i 21 p T 2 ( 1 - p 2
) w 1 i 12 p T 2 p 2 w 1 i 22 p T 2 ( 1 - p 2 ) w 2 i 12 p T 2 p 2
w 2 i 22 ] [ d 1 d 2 ] + [ n 1 1 n 2 1 n 1 2 n 2 2 ] = [ H 11 H 12
H 21 H 22 ] [ w i 11 w i 21 w i 12 w i 22 ] [ d 1 d 2 ] + [ n 1 n 2
] Equation 11 h ij mn ##EQU00011##
[0093] denotes a downlink channel between the j.sup.th transmission
antenna of the n.sup.th base station, and the i.sup.th reception
antenna of the m.sup.th terminal.
[0094] w.sub.j.sup.imn
[0095] denotes the j.sup.th component of a pre-coding vector mapped
to the i.sub.m n.sup.th index, which is suitable for a channel
between the n.sup.th base station and the m.sup.th terminal.
P.sub.T n denotes total transmit power of each antenna used in the
n.sup.th base station, and P.sub.n denotes a power control level
applied to the n.sup.th base station. And, n.sub.i.sup.m denotes
AWGN in the i.sup.th reception antenna of the m.sup.th
terminal.
[0096] {dot over (W)}
[0097] i.sub.m n of the Equation 11, which is for maximizing a
reception signal strength at a channel (H.sub.m n) between the
n.sup.th base station and the m.sup.th terminal is determined by
the following Equation 12. And, the terminal transmits a codebook
index (i.sub.m n) mapped to the
[0098] {dot over (W)}
[0099] i.sub.m n to the base station, thereby requesting a
pre-coding process.
i m n = argmax k = 1 , , L { H mn w . k 2 } Equation 12
##EQU00012##
[0100] A signal received by the m.sup.th terminal can be expressed
as the following Equation 13.
r m = [ h 11 m 1 h 12 m 1 h 11 m 2 h 12 m 2 h 21 m 1 h 22 m 1 h 21
m 2 h 22 m 2 ] [ p T 1 p 1 w 1 i 11 p T 1 ( 1 - p 1 ) w 1 i 21 p T
1 p 1 w 2 i 11 p T 1 ( 1 - p 1 ) w 2 i 21 p T 2 ( 1 - p 2 ) w 1 i
12 p T 2 p 2 w 1 i 22 p T 2 ( 1 - p 2 ) w 2 i 12 p T 2 p 2 w 2 i 22
] [ d 1 d 2 ] + [ n 1 m n 2 m ] = H ~ m [ w ~ 1 w ~ 2 ] d + n m = H
~ m w ~ m d m + k = 1 , k .noteq. m 2 H ~ m w ~ k d k + n m
Equation 13 ##EQU00013##
[0101] In the case of applying 1.times.N.sub.R reception signal
process vectors, u.sub.m=({tilde over (H)}.sub.m{tilde over
(W)}.sub.m).sup.H
[0102] using a selected pre-coding vector to the Equation 13, the
following Equation 14 is obtained.
u m r m = H ~ m w ~ m 2 d m + ( H ~ m w ~ m ) H ( k = 1 , k .noteq.
m 2 H ~ m w ~ k d k + n m ) = .lamda. m d m + n ' Equation 14
##EQU00014##
[0103] .lamda..sub.m denotes a beam forming gain obtained through
cooperative transmission using power control. And, a reception
Signal to Interference-plus-Noise Ratio (SINR) having AWGN signals
and multi-user interference signals applied thereto can be
expressed as the following Equation 15.
.gamma. = H ~ m w ~ m 2 k = 1 , k .noteq. m 2 H ~ m w ~ k d k + n m
2 Equation 15 ##EQU00015##
[0104] Differently from the conventional art, in the present
invention, the beam forming gain (.lamda..sub.m) is calculated
through distributed discrete power control, thereby implementing an
optimized closed loop MIMO system.
[0105] Hereinafter, the operation of an algorithm according to the
present invention will be explained in more detail.
[0106] In the case of applying the distributed discrete power
control (D.sup.2PC) of the present invention by using the existing
codebook in a cooperative system, transmission efficiency is
enhanced, and both the conventional non-cooperative transmission
and cooperative transmission can be executed.
[0107] A signal model of the present invention to which D.sup.2PC
has been applied can be expressed as the Equation 11. And, a
distributed discrete power control level (P.sub.n) used in the
n.sup.th base station, a core parameter of log.sub.2
[0108] Q
[0109] bit D.sup.2PC can be defined as a distributed discrete
level, which is shown in the following Equation 16 as an
example.
P Q = { p n | p n = q 2 ( Q - 1 ) + 1 2 , 0 .ltoreq. q .ltoreq. Q -
1 } Equation 16 ##EQU00016##
[0110] A D.sup.2PC level (P.sub.n) can be enhanced through
optimization in cooperative transmission environments. 1-bit, 2-bit
and 3-bit D.sup.2PC levels when the Q is 2, 4 and 8, respectively
can be expressed as follows.
P 2 = { 1 2 , 2 2 } ##EQU00017## P 4 = { 3 6 , 4 6 , 5 6 , 6 6 }
##EQU00017.2## P 8 = { 7 14 , 8 14 , 9 14 , , 14 14 }
##EQU00017.3##
[0111] In the case of applying
P 1 = P 2 = 1 2 ##EQU00018##
[0112] to the D.sup.2PC level of the Equation 11, the MIMO system
of the present invention operates in the same manner as the
conventional cooperative transmission method in a closed loop MIMO
system as shown in the following Equation 17.
r ~ = [ h 11 11 h 12 11 h 11 12 h 12 12 h 21 11 h 22 11 h 21 12 h
22 12 h 11 21 h 12 21 h 11 22 h 12 22 h 21 21 h 22 21 h 21 22 h 22
22 ] [ p T 1 p 1 w 1 i 11 p T 1 ( 1 - p 1 ) w 1 i 21 p T 1 p 1 w 2
i 11 p T 1 ( 1 - p 1 ) w 2 i 21 p T 2 ( 1 - p 2 ) w 1 i 12 p T 2 p
12 w 1 i 22 p T 2 ( 1 - p 2 ) w 2 i 12 p T 2 p 12 w 2 i 22 ] [ d 1
d 2 ] + [ n 1 1 n 2 1 n 1 2 n 2 2 ] = 1 2 [ h 11 11 h 12 11 h 11 12
h 12 12 h 21 11 h 22 11 h 21 12 h 22 12 h 11 21 h 12 21 h 11 22 h
12 22 h 21 21 h 22 21 h 21 22 h 22 22 ] [ p T 1 w 1 i 11 p T 1 w 1
i 21 p T 1 w 2 i 11 p T 1 w 2 i 21 p T 2 w 1 i 12 p T 2 w 1 i 22 p
T 2 w 2 i 12 p T 2 w 2 i 22 ] [ d 1 d 2 ] + [ n 1 1 n 2 1 n 1 2 n 2
2 ] Equation 17 ##EQU00019##
[0113] In the case of applying
P 1 = P 2 = 1 2 ##EQU00020##
[0114] to the D.sup.2PC level, the MIMO system of the present
invention operates in the same manner as the conventional
non-cooperative transmission method of a closed loop MIMO system as
shown in the following Equation 18.
r ~ = [ h 11 11 h 12 11 h 11 12 h 12 12 h 21 11 h 22 11 h 21 12 h
22 12 h 11 21 h 12 21 h 11 22 h 12 22 h 21 21 h 22 21 h 21 22 h 22
22 ] [ p T 1 w 1 i 11 0 p T 1 w 2 i 11 0 0 p T 2 w 1 i 22 0 p T 2 w
2 i 22 ] [ d 1 d 2 ] + [ n 1 1 n 2 1 n 1 2 n 2 2 ] Equation 18
##EQU00021##
[0115] That is, in the present invention, an optimized transmission
method can be implemented according to the value, P. And, the
transmission method of the present invention can operate as a
cooperative transmission method or a non-cooperative transmission
method.
[0116] As aforementioned, the transmission method of the present
invention can operate as a cooperative transmission method or a
non-cooperative transmission method according to a determined
D.sup.2PC level. Preferably, communication efficiency can be
maximized through optimization.
[0117] The D.sup.2PC is generally executed by the base station, but
may be also executed by the terminal.
[0118] Hereinafter, preferred embodiments of the present invention
will be explained according to the subject which executes the
D.sup.2PC.
[0119] In optimizing a reception signal vector through the
D.sup.2PC, the value of P has to be determined. Here, the D.sup.2PC
level may be determined by the base station according to one
embodiment of the present invention, or the D.sup.2PC level may be
determined by the terminal according to another embodiment of the
present invention.
[0120] FIG. 4 is a configuration view showing a method for
determining a distributed discrete power control level by the base
station according to one embodiment of the present invention.
[0121] Referring to FIG. 4, each terminal measures a reception
Signal to Interference-plus-Noise Ratio (SINR) and determines a
pre-coding index for communication (S110). A first terminal
estimates an SINR of a signal received from a second base station
through a channel. And, a second terminal estimates an SINR of a
signal received from a first base station through a channel.
[0122] Each terminal determines a pre-coding index capable of
maximizing a reception signal strength, by using power control
level information used in the base station, and channel estimation
information of a reception signal, based on channel information of
each base station participating in cooperative transmission.
[0123] Each terminal reports the measured SNR and the determined
pre-coding index to adjacent base stations (S120). In the case that
an uplink feedback channel is defined between one terminal and one
base station, only a base station having the most excellent channel
environment or which is the closest one may be reported with
feedback information.
[0124] The base stations may share the SNR and the pre-coding index
information transmitted from each terminal, with other base station
through a backbone network (S130). The sharing may be also
performed through an additional wired or wireless channel having a
good signal quality.
[0125] Through the sharing information between the base stations,
the base stations inform the pre-coding index of the first terminal
for multi-user transmission, to the second terminal grouped with
the first terminal (S140). The base station participating in
cooperative transmission shares pre-coding index information used
in one terminal having the most excellent channel environment or
located closest to the base station, with other terminals which
belong to a set grouped with the one terminal, based on a downlink
feedback channel.
[0126] Accordingly, each terminal simultaneously shares the
pre-coding index information used in other terminals. That is,
referring to FIG. 4, the first base station shares the pre-coding
index with the first terminal adjacent thereto, and the second base
station shares the pre-coding index with the second terminal
adjacent thereto.
[0127] Each base station determines a distributed discrete power
control (D2PC) level based on the information (S150).
[0128] The method for determining a D2PC level to be used in each
base station includes a method for allocating much power to a base
station having high reception signal power to be received by each
terminal, and a method for allocating the same signal power to each
base station by requesting a base station having low signal power
so as to have increased signal power.
[0129] In the case of allocating much power to a base stations
having high signal power, signal power is allocated in a similar
manner to a maximal ratio transmission method. In the case of
allocating much power to a base station having low signal power,
signal power is transmitted to each terminal from each base station
by the same amount (equal gain reception). And, a pre-coding vector
to be cooperatively transmitted from each base station operates so
as to be advantageous to orthogonality between interfering
pre-coding vectors.
[0130] The method for allocating much power to a base station
having high signal power to be received by each terminal will be
explained with reference to FIG. 6.
[0131] FIG. 6 shows diagrams for determining a distributed discrete
power control level.
[0132] Base stations participating in cooperative transmission
determine a distributed discrete power control level based on an
SINR of each terminal, and the diagrams of FIG. 6. For instance,
when it is assumed that SINR information between the n.sup.th base
station and the m.sup.th terminal reported from the m.sup.th
terminal is
[0133] .gamma..sub.mn
[0134] ,
[0135] {circumflex over (P)}
[0136] (x,y) for determining a distributed discrete power control
level can be defined as the following Equation 19.
p ^ ( x , y ) = p ^ ( .gamma. 11 .gamma. 12 , .gamma. 22 .gamma. 21
) Equation 19 ##EQU00022##
.gamma..sub.11
[0137] and
.gamma..sub.22
[0138] denote an SINR of a terminal having the most excellent
channel environment (or closest to a base station), which
satisfy
.gamma..sub.11
.gtoreq.
[0139] .gamma..sub.12
[0140] and
.gamma..sub.22
.gtoreq.
[0141] .gamma..sub.21
[0142] on the average. Accordingly,
.gamma..sub.11/.gamma..sub.12
[0143] and
.gamma..sub.22/.gamma..sub.21 have values more than 1. Based on
these characteristics, the diagrams shown in FIG. 6 can be
utilized.
[0144] Referring to FIG. 6, in an assumption that each terminal
knows all channels between BS.sub.1 (base station) and MS.sub.1
(mobile station: terminal), between BS.sub.1 and MS.sub.2, between
BS.sub.2 and MS.sub.1, and between BS.sub.2 and MS.sub.2, a
distributed discrete power control level is determined based on
four pre-coding indexes and two distributed discrete power control
levels to be transmitted from each base station to each
terminal.
.gamma..sub.11/.gamma..sub.12
[0145] ,
.gamma..sub.22/.gamma..sub.21
[0146] , and the distributed discrete power control levels when sum
rates of the MS.sub.1 and the MS.sub.2 are maximized are calculated
on the average, thereby determining a distributed discrete power
control value through the diagrams shown in FIG. 6.
[0147] When the
.gamma..sub.11/.gamma..sub.12
[0148] or
.gamma..sub.22/.gamma..sub.21
[0149] has a value less than 1, the terminal is located at a border
between cells. In this case, a distributed discrete power control
level of
[0150] {circumflex over (P)}
[0151] (x,y)=0 is determined to operate the MIMO system based on a
general cooperative transmission method. On the contrary, when
the
.gamma..sub.11/.gamma..sub.12
[0152] or
.gamma..sub.22/.gamma..sub.21
[0153] has a value more than 20, one or more terminals are located
at a border between cells. In this case, a distributed discrete
power control level of
[0154] {circumflex over (P)}
[0155] (x,y)=0 is determined to operate the MIMO system based on a
general cooperative transmission. On the contrary, when
.gamma..sub.11/.gamma..sub.12
[0156] or
.gamma..sub.22/.gamma..sub.21
[0157] has a value more than 20, one or more terminals are located
near the base stations. In this case, a distributed discrete power
control level of
[0158] {circumflex over (P)}
[0159] (x,y)=3 is determined to operate the MIMO system based on a
general non-cooperative transmission. In the case that each
terminal has two reception antennas and x=10 and y=6,
[0160] {circumflex over (p)}
[0161] (10,6)=2 is selected as a D.sup.2PC level by using the
diagrams shown in FIG. 6. In this manner, P.sub.n having been
determined by the n.sup.th base station is determined as a
distributed discrete power control level by using the diagrams
shown in FIG. 6.
[0162] Once the distributed discrete power control level has been
determined, the base stations report feedback information of the
distributed discrete power control level to the respective
terminals (S160).
[0163] In the above embodiment, the terminals report the SINR and
pre-coding index to all the adjacent base stations. However, this
procedure may be modified according to circumstances. For instance,
each terminal may report feedback information to one base station
closest thereto, and each base station may report the feedback
information to all the terminals. Alternatively, each terminal may
report feedback information to one base station closest thereto,
and each base station may report the feedback information to one
terminal closest thereto.
[0164] FIG. 5 is a configuration view showing a method for
determining a distributed discrete power control level by the
terminal according to another embodiment of the present
invention.
[0165] Referring to FIG. 5, each terminal measures a reception
Signal to Interference-plus-Noise Ratio (SINR) and determines a
pre-coding index for communication (S210). S210 is performed in the
same manner as S110 in FIG. 4.
[0166] Each terminal reports the measured SINR and the determined
pre-coding index to its adjacent base stations (S220). S220 is also
performed in the same manner as S120 in FIG. 4. The base stations
may share the SINR and the pre-coding index information transmitted
from each terminal, with other base stations through a backbone
network (S230).
[0167] Through the sharing information between the base stations,
the base stations share the pre-coding index with other terminals
grouped with each other for multi-user transmission in the same
manner as S140 in FIG. 4 (S240).
[0168] Each terminal determines a distributed discrete power
control level based on the pre-coding index information (S250).
[0169] The method for determining a distributed discrete power
control level shown in FIG. 5 is performed in the same manner as
the method for determining a distributed discrete power control
level shown in FIG. 4.
[0170] Once a distributed discrete power control level has been
determined, each terminal reports feedback information of the
determined distributed discrete power control level to each base
station (S260). Each terminal reports the determined distributed
discrete power control level to one base station having the most
excellent channel environment (the closest base station) through an
uplink channel. Each base station transmits a signal to the
terminal based on the distributed discrete power control level
reported from the terminal.
[0171] The distributed discrete power control level feedback
information reported from the terminal is shared with each base
station through a backbone network (S270). The sharing may be also
performed through an additional wired or wireless channel having a
good signal quality.
[0172] Each base station shares the determined distributed discrete
power control level feedback information with each terminal, based
on the shared distributed discrete power control level feedback
information (S280). That is, the distributed discrete power control
level feedback information is shared with terminals belonging to a
multi-user group which simultaneously receives a signal through a
cooperative multi-user transmission method. The base station
belonging to cooperative transmission transmits distributed
discrete power control level information used by other base
stations belonging to the cooperative transmission, to one terminal
having the most excellent channel environment (i.e., the closest
terminal) through a downlink channel. Through these procedures,
each terminal can be provided with distributed discrete power
control levels used in all the base stations belonging to
cooperative transmission. And, the information is utilized for data
restoration using a reception signal of the terminal.
[0173] In the above embodiment, the terminals report the SINR and
pre-coding index to all the adjacent base stations. However, this
procedure may be modified according to circumstances. For instance,
each terminal may report feedback information to a base station
closest thereto.
[0174] For explanations of effects of the present invention,
simulations have been performed.
[0175] For the simulations, a multi-cell mobile communication
system having M.times.N=2.times.4 channels was assumed. In the
multi cell mobile communication system, two base stations (B=2)
each having two antennas (N.sub.T=2), and two terminals (K=2) each
having one antenna (N.sub.R=1) were used, and 19 cells were
used.
[0176] In the present experiments, each base station that
determines the distributed discrete power control level. The same
total power, P.sub.T 1=P.sub.T 2=1 is allocated to the antennas of
each base station. And, it is assumed that all the base stations
have the same distributed discrete power control level
(P.sub.1=P.sub.2).
[0177] As a pre-coding technique applied to the simulations, a
Discrete Fourier Transform base codebook; DFT-based codebook is
used.
[0178] The DFT-based codebook is utilized to generate a pre-coding
vector used to apply a beam forming technique in a closed loop MIMO
system, and may be generated by applying a pre-coding vector
(W.sub.K) to be used in N.sub.T transmission antennas to a Discrete
Fourier Transform Matrix (
W.sub.M.sup.NT
[0179] ).
[0180] FIG. 7 is a view comparing MIMO system capacities according
to the position of the terminal according to the present
invention.
[0181] Referring to FIG. 7, in an assumption that a distance
between a base station and the m.sup.th terminal is d.sub.m, in the
case that the terminal is positioned on a straight line between a
base station position (d.sub.m=0) and a cell border position
(d.sub.m=1), MIMO system capacities were measured. The MIMO system
capacities were measured according to three positions (L1, L2 and
L3) of the two terminals. Here, `L1` denotes a position where the
two terminals are located at a cell border, and `L2` denotes a
position where one terminal is located at a cell border whereas
another terminal is located near the base station. And, `L3`
denotes a position where the two terminals are located near the
base stations.
[0182] Referring to the table of FIG. 7, "2+2" signifies that each
base station has used two bits for one terminal, and thus four bits
has been totally used. And, "2+2+1" or "2+2+2" signifies that each
base station has used two bits for one terminal and thus four bits
has been totally used, and a distributed discrete power control
level having 1 or 2-bit has been used.
[0183] As can be seen from the table of FIG. 7, in the case of
using the distributed discrete power control, MIMO system capacity
of the present invention was enhanced more than the conventional
MIMO system capacity based on a non-cooperative transmission (NCT)
and cooperative transmission (CT). Especially, the MIMO system
capacity was similar to that based on cooperative transmission in
the `L1`, and was similar to that based on non-cooperative
transmission in the `L3`.
[0184] FIG. 8 is a view comparing MIMO system capacities when two
terminals are constantly disposed within each coverage of two base
stations participating in cooperative transmission.
[0185] FIG. 8 shows sum rates of two terminals having an SINR less
than .lamda..sub.t h=3 d B (an outage threshold) and receiving
signals from two base stations through a cooperative transmission
method.
[0186] As can be seen from FIG. 8, in the case of using the
distributed discrete power control method of the present invention,
the MIMO system capacity was enhanced in all the terminals. And,
the terminals having a reception rate of 20% have obtained the sum
rate gains more than 0.4 bps/Hz.
[0187] The foregoing embodiments and advantages are merely
exemplary and are not to be construed as limiting the present
disclosure. The present teachings can be readily applied to other
types of apparatuses. This description is intended to be
illustrative, and not to limit the scope of the claims. Many
alternatives, modifications, and variations will be apparent to
those skilled in the art. The features, structures, methods, and
other characteristics of the exemplary embodiments described herein
may be combined in various ways to obtain additional and/or
alternative exemplary embodiments.
[0188] As the present features may be embodied in several forms
without departing from the characteristics thereof, it should also
be understood that the above-described embodiments are not limited
by any of the details of the foregoing description, unless
otherwise specified, but rather should be construed broadly within
its scope as defined in the appended claims, and therefore all
changes and modifications that fall within the metes and bounds of
the claims, or equivalents of such metes and bounds are therefore
intended to be embraced by the appended claims.
* * * * *